1. Field of the Invention
This invention is directed to methods for sterilizing cross-linked gelatin as well as to sterilized cross-linked gelatin compositions possessing novel properties. In particular, the methods of this invention employ E-beam irradiation to sterilize cross-linked gelatin.
2. References
The following patent applications and patents are cited and/or reference in this application as superscript numbers:                1 Correll, et al., Proc. Soc. Exp. Biol. N.Y., 58:233 (1945)        2 Correll, et al., Surg. Gyn. and Obst., 82:585 (1945)        3 Correll, et al., U.S. Pat. No. 2,465,357, Therapeutic Sponge and Method of Making, issued Mar. 29, 1949        4 Correll, et al., U.S. Pat. No. 2,507,244, Surgical Gelatin Dusting Powder and Process for Preparing Same, issued May 9, 1950        5 Studer, et al., U.S. Pat. No. 2,558,395, Undenatured Gelatin Hemostatic Sponge Containing Thrombin, issued Jun. 26, 1951        6 Sieger, et al., U.S. Pat. No. 2,899,362, Hemostatic Sponges and Method of Preparing Same, issued Aug. 11, 1959        7 Song, et al., U.S. Pat. No. 5,399,361, Collagen-containing Sponges as Drug Delivery Compositions for Proteins, issued Mar. 21 1995        8 Cragg, et al., U.S. Pat. No. 6,071,301, Device and Method for Facilitating Hemostasis of a Biopsy Tract, issued Jun. 6, 2000        9 Cragg, et al., U.S. Pat. No. 6,086,607, Device and Method for Facilitating Hemostasis of a Biopsy Tract, issued Jul. 11, 2000        10 Cragg, et al., U.S. Pat. No. 6,162,192, System and Method for Facilitating Hemostasis of Blood Vessel Punctures with Absorbable Sponge, issued Dec. 19, 2000        11 Pawelchak, et al., U.S. Pat. No. 4,292,972, Lyophilized Hydrocolloid Foam, issued Oct. 6, 1981        12 Sawyer, U.S. Pat. No. 4,238,480, Method for Preparing an Improved Hemostatic Agent and Method of Employing the Same, issued Dec. 9, 1980        13 Sawyer, U.S. Pat. No. 4,404,970, Hemostatic Article and Method for Preparing and Employing the Same, issued Sep. 20, 1983        
All of the above patent applications and patents are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent application or patent was specifically and individually indicated to be incorporated by reference in its entirety.
3. State of the Art
Cross-linked gelatin, often in the form of gelatin foam, gelatin film or gelatin sponges, has been used as a hemostatic agent since its development by Correll in 1945.1-4 In addition, medicaments, such as antibiotics, growth factors and thrombus enhancing agents, have been incorporated into the cross-linked gelatin to enhance the in vivo properties of the composition.5-7 
When used as a hemostatic agent, the cross-linked gelatin is placed on or in the body and, accordingly, the composition must be sterilized before use, Conventionally, sterilization of these cross-linked gelatin compositions is conducted at elevated temperatures for prolonged periods of time, e.g., 130° to 140° C. for 3 hours as described by Correll.3 While the resulting cross-linked gelatin composition is sterile, the sterilization process causes chemical reactions within the cross-linked gelatin (polypeptide) which results in hardening and insolubilization of the gelatin. These changes can be correlated with the tensile strength and fluid (e.g., water, blood, etc.) uptake of the cross-linked gelatin composition before and after heat sterilization and the heat sterilized product has higher tensile strength and significantly less fluid uptake as compared to the pre-sterilized product.
One particular use of cross-linked gelatin described in the art is to facilitate hemostasis of a puncture site such as a puncture wound resulting from catheter insertion or a biopsy needle. When so used, the art describes ejection of a pledget of cross-linked gelatin from a syringe into the puncture site.8-10 
Critical to the ejection process is the flowability of the pledget from the syringe assembly and retention of its structural integrity during insertion into the body. Specifically, ejection of the pledget from the syringe assembly is preferably conducted with, at most, moderate pressure to ensure accurate placement in vivo which relates to the flowability of the cross-linked gelatin. Higher fluid content pledgets are believed to correlate with enhance flowability and, accordingly, it is desirable to maintain as high a fluid absorbability content in the sterilized pledget as possible.
Likewise, it is critical that the structural integrity of the pledget is substantially maintained as it is ejected from the syringe assembly when placed in vivo in order to ensure that portions of the pledget are not torn or otherwise separated from the pledget. This criticality is particularly important when placed over a blood vessel puncture in order to avoid unintended thrombosis of the vessel. Structural integrity of the gelatin composition of the pledget under pressure is believed to correlate with the tensile strength of the composition and, accordingly, it is desirable to maintain as high a tensile strength in the sterilized pledget as possible.
However, as demonstrated in the Examples below, the heat sterilization processes of the prior art significantly reduce the water absorbability of the cross-linked gelatin and only modestly increase its tensile strength. In addition, heat sterilized gelatin has a significant drawback when packaged within a device for use because dry heat sterilization requires prolonged heating at elevated temperatures (e.g., 140° C. for 8 hours for a metal component to be sterilized). Notwithstanding such prolonged heating, these processes are often ineffective in reducing bioburden to a level recognized to effect sterility. The possibility of ethylene oxide or gamma (γ) sterilization of gelatin has also been mentioned in the literature.11-13 However, these methods cause irreversible reactions within the gelatin leading to altered and possibly undersirable physical and biological properties.
As is apparent, methods for sterilizing cross-linked gelatin compositions without significant reductions in fluid absorbability while significantly increasing its tensile strength would be of great value. In addition, it would be particularly valuable if such methods would sterilize packaged gelatin compositions, such as those contained within finished medical devices such as a delivery system of syringes, syringes, or other assemblies in order to facilitate manufacture of sterile devices.